On 12-08-2008 at 14:03, Anonymous wrote:
The oscillator frequency spec is stated as 1% (at 25C).
The oscillator can meet this spec if
A) the frequency varies randomly due to lots of jitter by +-.25%, on a cycle-by-cycle basis, effectively acting as a 'spread spectrum' clock. On top of this is added thermal and voltage drift.
Every second the frequency will be wobbling around randomly over a 40KHz range.
B) The frequency over 1s varies by 1KHz, and changes due to thermal and voltage effects.
In the case of B) as I understand it, it might be plausible to first calibrate the clock speed using the RTC or other means, then use the USB device - for few seconds. (I'd not assume trimming with USB in use would work without more info on the PLL)
This is not solely an issue with USB of course.
It makes the HSI useless for some tasks involving relative timing, when the drift alone isn't a problem.
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